While guided wave optical systems have become commonplace in telecommunications, such systems exhibit several practical limitations. Installation of such systems in existing buildings can be disruptive and labor intensive, and in some cases cannot be economically configured to reach all building areas for which optical communication services are desired. Communication applications between two or more buildings or other locations typically require underground installation of optical cables in existing utility tunnels or burial in newly dug trenches. Installation in either of these manners can be slow and expensive. In addition, reconfiguration of a guided wave communication system can be difficult as additional optical fiber or other waveguide must be installed to reach any newly selected locations. As a result, an installed communication system can require expensive upgrades.
Free space optical communication systems do not exhibit the limitations associated with the installation and maintenance of guided wave optical communication systems. Such systems include optical transmitters and receivers that are configured to deliver and receive optical signals propagating in free space, and waveguides are not needed to connect the transmitter and receiver. In addition, such transmitters and receivers can be portable and readily adapted to a user's varying communication requirements.
While free space optical communication systems have numerous advantages, such systems typically have several drawbacks. Transmitter and receiver locations can be easily changed but a transmitter and an associated receiver must be located within a line of sight. In communication between existing commercial buildings or other structures, such transmitters/receivers are conveniently placed near windows, but frequently block unacceptably large portions of the window. In addition, free space optical systems are also susceptible to light from sources other than the desired transmitter. For example, sunlight or room illumination can be detected and interfere with a modulated optical carrier that contains a desired data stream. Free space optical systems are also subject to mechanical disturbances because they are frequently placed in exposed locations. For these reasons, improved free space optical communication systems, methods, and apparatus are needed.